Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having a decreased spin rate on driver shots. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) a compound represented by the formula (1) as a co-crosslinking agent, and (c) a crosslinking initiator, 
       (R 1 COO)M(OCOR 2 ) m   (1)
         in the formula (1), M represents a metal atom, m represents 1 or 2, R 1  and R 2  are different from each other, represent an alkenyl group having 2 to 30 carbon atoms or an alkynyl group having 2 to 30 carbon atoms and have a carbon-carbon double bond or a carbon-carbon triple bond at an α-position carbon atom bonding to the carbonyl group, and when m is 2, two of R 2  may be identical to or different from each other.

FIELD OF THE INVENTION

The present invention relates to a golf ball having excellent flightperformance, and more specifically relates to a technology for improvinga core rubber composition of a golf ball.

DESCRIPTION OF THE RELATED ART

As a material for forming a core of a golf ball, a rubber compositioncontaining a base rubber, a co-crosslinking agent and a crosslinkinginitiator is widely used in light of its good resilience.

For example, JP 2018-102693 A discloses a golf ball comprising aspherical core and at least one cover layer covering the spherical core,wherein the spherical core is formed from a rubber compositioncontaining (a) a base rubber, (b) a co-crosslinking agent, and (c) acrosslinking initiator, and (b) the co-crosslinking agent contains acompound represented by a formula (1):

(R¹OCO)M(COOR²)m  (1)

in the formula (1), M represents a metal atom, and m represents 1 or 2wherein when m is 1, R¹ and R² are different from each other andrepresent an alkenyl group having 2 to 30 carbon atoms or an alkynylgroup having 2 to 30 carbon atoms; when m is 2, R¹ represents an alkenylgroup having 2 to 30 carbon atoms, an alkynyl group having 2 to 30carbon atoms, an alkyl group having 1 to 30 carbon atoms, or an arylgroup having 6 to 30 carbon atoms, two of R² are identical to ordifferent from each other and represent an alkenyl group having 2 to 30carbon atoms or an alkynyl group having 2 to 30 carbon atoms; and when mis 2, a compound in which R¹ and two of R² are all identical to eachother is excluded.

JP 2018-86179 A discloses a golf ball having a crosslinked moldedproduct of a rubber composition as a constituent element, wherein therubber composition contains the following components (a) to (d): (a) abase rubber, (b) an α,β-unsaturated carboxylic acid and/or a metal saltthereof, (c) a crosslinking initiator, and (d) a carboxylic acid metalsalt in which the carboxylic acid bonded to the metal is of two or moredifferent types and at least one of the carboxylic acids has 8 or morecarbon atoms.

US 2018-002510 A discloses a golf ball comprising a core, a cover, andan intermediate layer disposed between the core and the cover, whereinthe core contains a blend of a polybutadiene rubber and a fatty acid(meth)acrylic acid salt, where the fatty acid (meth)acrylic acid saltincludes a reaction product of a fatty acid, a (meth)acrylic acidmonomer, and M(OH)x or MxOy, where M is a metal cation, and x and yindependently range from about 1 to about 7, and where the fatty acid(meth)acrylic acid salt is present in the blend in an amount of about 1phr to about 70 phr.

SUMMARY OF THE INVENTION

In a conventional golf ball, zinc acrylate is used as a co-crosslinkingagent. The resilience of the golf ball is improved by using zincacrylate. However, there is a limit of improving the resilience of agolf ball using zinc acrylate. Thus, it is desired to improve a flightdistance of a golf ball by a different method. On the other hand, it isknown that the flight distance on driver shots increases if the spinrate on driver shots is decreased. The present invention has been madein view of the above circumstances, and an object of the presentinvention is to provide a golf ball having a decreased spin rate ondriver shots.

The present invention that has solved the above problem provides a golfball comprising a spherical core and at least one cover layer coveringthe spherical core, wherein the spherical core is formed from a rubbercomposition containing (a) a base rubber, (b) a co-crosslinking agent,and (c) a crosslinking initiator, and (b) the co-crosslinking agentcontains a compound represented by the formula (1):

(R¹OCO)M(COOR²)m  (1)

in the formula (1), M represents a metal atom, m represents 1 or 2, R¹and R² are different from each other, represent an alkenyl group having2 to 30 carbon atoms or an alkynyl group having 2 to 30 carbon atoms andhave a carbon-carbon double bond or a carbon-carbon triple bond at anα-position carbon atom bonding to a carbonyl group, and when m is 2, twoof R² may be identical to or different from each other.

According to the present invention, a golf ball having a decreased spinrate on driver shots is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention that has solved the above problem provides a golfball comprises a spherical core and at least one cover layer coveringthe spherical core, wherein the spherical core is formed from a rubbercomposition containing (a) a base rubber, (b) a co-crosslinking agent,and (c) a crosslinking initiator, and (b) the co-crosslinking agentcontains a compound represented by the formula (1):

(R¹OCO)M(COOR²)m  (1)

in the formula (1), M represents a metal atom, m represents 1 or 2, R¹and R² are different from each other, represent an alkenyl group having2 to 30 carbon atoms or an alkynyl group having 2 to 30 carbon atoms andhave a carbon-carbon double bond or a carbon-carbon triple bond at anα-position carbon atom bonding to a carbonyl group, and when m is 2, twoof R² may be identical to or different from each other.

[Spherical Core]

The spherical core is formed from a rubber composition containing (a) abase rubber, (b) a co-crosslinking agent and (c) a crosslinkinginitiator.

((a) Base Rubber)

As (a) the base rubber, a natural rubber and/or a synthetic rubber canbe used. For example, a polybutadiene rubber, a natural rubber, apolyisoprene rubber, a styrene polybutadiene rubber, or anethylene-propylene-diene rubber (EPDM) can be used. These rubbers may beused solely, or at least two of these rubbers may be used incombination. Among them, particularly preferred is a high-cispolybutadiene having a cis-1,4 bond in an amount of 40 mass % or more,preferably 80 mass % or more, and more preferably 90 mass % or more inview of their superior resilience. The amount of the high-cispolybutadiene in (a) the base rubber is preferably 50 mass % or more,more preferably 70 mass % or more.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in an amountof 2 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. If the amount of the 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cis polybutadiene is preferably a polybutadiene synthesizedusing a rare earth element catalyst. When a neodymium catalyst, whichemploys a neodymium compound that is a lanthanum series rare earthelement compound, is used, a polybutadiene rubber having a high contentof a cis-1,4 bond and a low content of a 1,2-vinyl bond is obtained withexcellent polymerization activity. Such a polybutadiene rubber isparticularly preferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or more, more preferably 32 or more, even morepreferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140 or less, more preferably 120 or less, even morepreferably 100 or less, and most preferably 80 or less. It is noted thatthe Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is avalue measured according to JIS K6300-1 (2013) using an L rotor underthe conditions of: a preheating time of 1 minute; a rotor revolutiontime of 4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cis polybutadiene is excessively low, the processabilitydeteriorates. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, available from Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(available from Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

((b) Co-Crosslinking Agent)

(b) The co-crosslinking agent contains a compound represented by theformula (1). If the compound represented by the formula (1) is blendedas the co-crosslinking agent, the spin rate on driver shots is lowered.

(R¹OCO)M(COOR²)m  (1)

In the formula (1), M represents a metal atom, m represents 1 or 2, R¹and R² are different from each other, represent an alkenyl group having2 to 30 carbon atoms or an alkynyl group having 2 to 30 carbon atoms andhave a carbon-carbon double bond or a carbon-carbon triple bond at anα-position carbon atom bonding to a carbonyl group, and when m is 2, twoof R² may be identical to or different from each other.

Examples of the metal atom (M) include an alkaline earth metal such ascalcium, strontium, and barium; a transition metal such as scandium,titanium, vanadium, chrome, manganese, iron, cobalt, nickel, copper,yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium,palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, indium,platinum, and gold; and a base metal such as beryllium, magnesium,aluminum, zinc, gallium, cadmium, indium, tin, thallium, lead, bismuth,and polonium. The metal atom may be used solely, or two or more of themmay be used in combination. Among them, as the metal atom, the metalatom forming a divalent or trivalent metal ion is preferable, at leastone metal atom selected from the group consisting of beryllium,magnesium, calcium, zinc, barium, cadmium, lead and aluminum is morepreferable.

The alkenyl group having 2 to 30 carbon atoms may be linear or branched,and is preferably linear. The alkenyl group has the carbon-carbon doublebond at an α-position carbon atom bonding to the carbonyl group in theformula (1).

The alkenyl group having 2 to 30 carbon atoms preferably has at leastone carbon-carbon double bond. Examples of the alkenyl group having 2 to30 carbon atoms include an alkenyl group having one carbon-carbon doublebond, and an alkenyl group having two carbon-carbon double bonds. Thetwo carbon-carbon double bonds of the alkenyl group having twocarbon-carbon double bonds are preferably conjugated.

The number of carbon atoms included in the alkenyl group is preferably 2or more, and is preferably 30 or less, more preferably 20 or less, andeven more preferably 10 or less.

Examples of the alkenyl group having 2 to 30 carbon atoms and having onecarbon-carbon double bond include ethenyl group (C2: vinyl group),1-propenyl group (C3), isopropenyl group (C3), 1-butenyl group (C4),1-pentenyl group (C5), 1-hexenyl group (C6), 1-heptenyl group (C7),1-octenyl group (C8), 1-nonenyl group (C9), 1-decenyl group (C10),1-undecenyl group (C11), 1-dodecenyl group (C12), 1-tridecenyl group(C13), 1-tetradecenyl group (C14), 1-pentadecenyl group (C15),1-hexadecenyl group (C16), 1-heptadecenyl group (C17), 1-octadecenylgroup (C18), 1-nonadecenyl group (C19), 1-icocenyl group (C20),1-henicosenyl group (C21), 1-docosenyl group (C22), 1-tricosenyl group(C23), 1-tetracosenyl group (C24), 1-pentacosenyl group (C25),1-hexacosenyl group (C26), 1-heptacosenyl group (C27), 1-octacosenylgroup (C28), 1-nonacosenyl group (C29), and 1-triacontenyl group (C30).

Examples of the alkenyl group having 4 to 30 carbon atoms and having twocarbon-carbon double bonds include 1,3-butadienyl group (C4),1,3-pentadienyl group (C5), 1,3-hexadienyl group (C6), 1,3-heptadienylgroup (C7), 1,3-octadienyl group (C8), 1,3-nonadienyl group (C9),1,3-decadienyl group (C10), 1,3-undecadienyl group (C11),1,3-dodecadienyl group (C12), 1,3-tridecadienyl group (C13),1,3-tetradecadienyl group (C14), 1,3-pentadecadienyl group (C15),1,3-hexadecadienyl group (C16), 1,3-heptadecadienyl group (C17),1,3-octadecadienyl group (C18), 1,3-nonadecadienyl group (C19),1,3-icosadienyl group (C20), 1,3-henicosadienyl group (C21),1,3-docosadienyl group (C22), 1,3-trcosadienyl group (C23),1,3-tetracosadienyl group (C24), 1,3-pentacosadienyl group (C25),1,3-hexacosadienyl group (C26), 1,3-heptacosadienyl group (C27),1,3-octacosadienyl group (C28), 1,3-nonacosadienyl group (C29), and1,3-tracontadienyl group (C30).

As the alkenyl group having the carbon-carbon double bond, for example,ethenyl group (C2: vinyl group), 1-propenyl group (C3), isopropenylgroup (C3), 1,3-butadienyl group (C4) or 1,3-pentadienyl group (C5) ispreferable.

The alkynyl group having 2 to 30 carbon atoms may be linear or branched,and is preferably linear. The alkynyl group having 2 to 30 carbon atomshas a carbon-carbon triple bond at the α-position carbon atom bonding tothe carbonyl group in the formula (1). The alkynyl group having 2 to 30carbon atoms preferably has at least one carbon-carbon triple bond.Examples of the alkynyl group having 2 to 30 carbon atoms include analkynyl group having one carbon-carbon triple bond, and an alkynyl grouphaving two carbon-carbon triple bonds.

The number of carbon atoms included in the alkynyl group is preferably 2or more, and is preferably 30 or less, more preferably 20 or less, andeven more preferably 10 or less.

Examples of the alkynyl group having 2 to 30 carbon atoms includeethynyl group (C2), 1-propynyl group (C3), 1-butynyl group (C4),1-pentynyl group (C5), 1-hexynyl group (C6), 1-heptynyl group (C7),1-octynyl group (C8), 1-nonynyl group (C9), 1-decynyl group (C10),1-undecynyl group (C11), 1-dodecynyl group (C12), 1-tridecynyl group(C13), 1-tetradecynyl group (C14), 1-pentadecynyl group (C15),1-hexadecynyl group (C16), 1-heptadecynyl group (C17), 1-octadecynylgroup (C18), 1-nonadecynyl group (C19), 1-icosynyl group (C20),1-henicosynyl group (C21), 1-docosynyl group (C22), 1-tricosynyl group(C23), 1-tetracosynyl group (C24), 1-pentacosynyl group (C25),1-hexacosynyl group (C26), 1-heptacosynyl group (C27), 1-octacosynylgroup (C28), 1-nonacosynyl group (C29), and 1-triacontynyl group (C30).

Examples of the alkynyl group having the carbon-carbon triple bondinclude ethynyl group (C2), 1-propynyl group (C3), and 1-butynyl group(C4).

In the compound represented by the formula (1), the number of carbonatoms included in R¹ is preferably 2 or more, and is preferably 6 orless, more preferably 3 or less. As R¹, ethenyl group (vinyl group) orisopropenyl group is preferable, ethenyl group (vinyl group) is morepreferable.

In the compound represented by the formula (1), the number of carbonatoms included in R² is preferably 2 or more, more preferably 3 or more,and is preferably 6 or less. As R², 1-propenyl group, 1,3-butadienylgroup or 1,3-pentadienyl group is preferable, 1-propenyl group or1,3-butadienyl group is more preferable, and 1-propenyl group is evenmore preferable.

Specific examples of the compound represented by the formula (1) includea compound in which m=1, R¹=ethenyl group (vinyl group) andR²=1-propenyl group; a compound in which m=1, R¹=ethenyl group (vinylgroup) and R²=1,3-butadienyl group; a compound in which m=1, R¹=ethenylgroup (vinyl group) and R²=1,3-pentadienyl group.

[Preparation of the Compound Represented by the Formula (1)]

The compound represented by the formula (1) can be obtained by areaction between a carboxylic acid and a metal oxide. Specific examplesof the preparation method include a method of mixing a metal oxide, andat least a first carboxylic acid and a second carboxylic acid in asolvent.

Examples of the metal oxide include an oxide of an alkaline earth metal,such as calcium oxide, strontium oxide, and barium oxide; an oxide of atransition metal, such as scandium oxide, titanium oxide, vanadiumoxide, chrome oxide, manganese oxide, iron oxide, cobalt oxide, nickeloxide, copper oxide, yttrium oxide, zirconium oxide, niobium oxide,molybdenum oxide, technetium oxide, ruthenium oxide, rhodium oxide,palladium oxide, silver oxide, hafnium oxide, tantalum oxide, tungstenoxide, rhenium oxide, osmium oxide, iridium oxide, platinum oxide, andgold oxide; and an oxide of a base metal, such as beryllium oxide,magnesium oxide, aluminum oxide, zinc oxide, gallium oxide, cadmiumoxide, indium oxide, tin oxide, thallium oxide, lead oxide, bismuthoxide, and polonium oxide. The metal oxide may be used solely, or two ormore of them may be used in combination. Among them, as the metal oxide,the oxide of the divalent metal is preferable, beryllium oxide,magnesium oxide, calcium oxide, zinc oxide, barium oxide, cadmium oxideor lead oxide is more preferable.

Examples of the carboxylic acid include an unsaturated fatty acid having3 to 30 carbon atoms. Examples of the unsaturated fatty acid include anunsaturated fatty acid having a carbon-carbon double bond, and anunsaturated fatty acid having a carbon-carbon triple bond. Thecarbon-carbon double bond or carbon-carbon triple bond is preferably atthe α-position (2-position) carbon atom bonding to the carboxyl group.

The unsaturated fatty acid having the carbon-carbon double bondpreferably has at least one carbon-carbon double bond. Examples of theunsaturated fatty acid having the carbon-carbon double bond include anunsaturated fatty acid having one carbon-carbon double bond, and anunsaturated fatty acid having two carbon-carbon double bonds. The twocarbon-carbon double bonds of the unsaturated fatty acid having twocarbon-carbon double bonds is preferably conjugated.

The number of carbon atoms included in the unsaturated fatty acid havingthe carbon-carbon double bond is preferably 3 or more, and is preferably30 or less, more preferably 20 or less, and even more preferably 10 orless.

Examples of the unsaturated fatty acid having one carbon-carbon doublebond include 2-propenoic acid (C3: acrylic acid), 2-methyl-2-propenoicacid (C4: methacrylic acid), 2-butenoic acid (C4: crotonic acid),2-pentenoic acid (C5), 2-hexenoic acid (C6), 2-heptenoic acid (C7),2-octenoic acid (C8), 2-nonenoic acid (C9), 2-decenoic acid (C10),2-undecenoic acid (C11), 2-dodecenoic acid (C12), 2-tridecenoic acid(C13), 2-tetradecenoic acid (C14), 2-pentadecenoic acid (C15),2-hexadecenoic acid (C16), 2-heptadecenoic acid (C17), 2-octadecenoicacid (C18), 2-nonadecenoic acid (C19), 2-icosenoic acid (C20),2-henicosenoic acid (C21), 2-docosenoic acid (C22), 2-tricosenoic acid(C23), 2-tetracosenoic acid (C24), 2-pentacosenoic acid (C25),2-hexacosenoic acid (C26), 2-heptacosenoic acid (C27), 2-octacosenoicacid (C28), 2-nonacosenoic acid (C29), and 2-triacontenoic acid (C30).

Examples of the unsaturated fatty acid having two carbon-carbon doublebonds include butadienoic acid (C4), pentadienoic acid (C5), hexadienoicacid (C6), heptadienoic acid (C7), octadienoic acid (C8), nonadienoicacid (C9), decadienoic acid (C10), undecadienoic acid (C11),dodecadienoic acid (C12), tridecadienoic acid (C13), tetradecadienoicacid (C14), pentadecadienoic acid (C15), hexadecadienoic acid (C16),heptadecadienoic acid (C17), octadecadienoic acid (C18), nonadecadienoicacid (C19), icosadienoic acid (C20), henicosadienoic acid (C21),docosadienoic acid (C22), tricosadienoic acid (C23), tetracosadienoicacid (C24), pentacosadienoic acid (C25), hexacosadienoic acid (C26),heptacosadienoic acid (C27), octacosadienoic acid (C28), nonacosadienoicacid (C29), and triacontadienoic acid (C30).

The two carbon-carbon double bonds are preferably conjugated. In otherwords, a 2,4-unsaturated carboxylic acid having carbon-carbon doublebonds at carbon atoms at 2-position and 4-position is preferable.

The unsaturated carboxylic acid having the carbon-carbon triple bondpreferably has at least one carbon-carbon triple bond. Examples of theunsaturated fatty acid having the carbon-carbon triple bond include anunsaturated fatty acid having one carbon-carbon triple bond, and anunsaturated fatty acid having two carbon-carbon triple bonds.

The number of carbon atoms included in the unsaturated fatty acid havingthe carbon-carbon triple bond is preferably 3 or more, and is preferably30 or less, more preferably 20 or less, and even more preferably 10 orless.

Examples of the unsaturated fatty acid having one carbon-carbon triplebond include 2-propiolic acid (C3), 2-butynoic acid (C4), 2-pentynoicacid (C5), 2-hexynoic acid (C6), 2-heptynoic acid (C7), 2-octynoic acid(C8), 2-nonynoic acid (C9), 2-decynoic acid (C10), 2-undecynoic acid(C11), 2-dodecynoic acid (C12), 2-tridecynoic acid (C13),2-tetradecynoic acid (C14), 2-pentadecynoic acid (C15), 2-hexadecynoicacid (C16), 2-heptadecynoic acid (C17), 2-octadecynoic acid (C18),2-nonadecynoic acid (C19), 2-icosynoic acid (C20), 2-heneicosynoic acid(C21), 2-docosynoic acid (C22), 2-tricosynoic acid (C23),2-tetracosynoic acid (C24), 2-pentacosynoic acid (C25), 2-hexacosynoicacid (C26), 2-heptacosynoic acid (C27), 2-octacosynoic acid (C28),2-nonacosynoic acid (C29), and 2-triacontynoic acid (C30).

In the present invention, as the first carboxylic acid, 2-propenoic acid(acrylic acid) or 2-methyl-2-propenoic acid (methacrylic acid) ispreferably used, and 2-propenoic acid (acrylic acid) is more preferablyused. In addition, as the second carboxylic acid, 2-butenoic acid(crotonic acid), 2,4-pentadienoic acid or 2,4-hexadienoic acid (sorbicacid) is preferably used, and 2-butenoic acid (crotonic acid) or2,4-pentadienoic acid is more preferably used.

The amounts of the first carboxylic acid and the second carboxylic acidcan be appropriately adjusted depending on the chemical structure of thedesired fatty acid metal salt. It is noted that if a product obtained bymultiplying the moles of the metal ion in the metal oxide by the valenceof this metal ion (a sum of a product obtained by multiplying the molesof each metal ion by the valence of this metal ion when a plurality ofmetal ions are present) is referred to as X, the moles of the carboxygroup in the first carboxylic acid is referred to as Y₁, and the molesof the carboxy group in the second carboxylic acid is referred to as Y₂,the ratio ((Y₁+Y₂)/X) is 0 or more, and is preferably 1.0 or less.

Examples of the solvent include toluene and xylene. The amount of thesolvent is preferably 150 mL or more, more preferably 200 mL or more,and is preferably 600 mL or less, more preferably 500 mL or less, withrespect to 100 g of the total amount of the carboxylic acids.

The liquid temperature when mixing the metal oxide and the carboxylicacids is preferably 10° C. or more, more preferably 15° C. or more, andis preferably 100° C. or less, more preferably 90° C. or less. Themixing time when mixing the metal oxide and the carboxylic acids can beappropriately adjusted.

When a third carboxylic acid is used as the carboxylic acid in additionto the first carboxylic acid and the second carboxylic acid, the amountsof the first carboxylic acid, the second carboxylic acid and the thirdcarboxylic acid can be appropriately adjusted depending on the chemicalstructure of the desired fatty acid metal salt. It is noted that if aproduct obtained by multiplying the moles of the metal ion in the metaloxide by the valence of this metal ion (a sum of a product obtained bymultiplying the moles of each metal ion by the valence of this metal ionwhen a plurality of metal ions are present) is referred to as X, themoles of the carboxy group in the first carboxylic acid is referred toas Y₁, the moles of the carboxy group in the second carboxylic acid isreferred to as Y₂, and the moles of the carboxy group in the thirdcarboxylic acid is referred to as Ys, the ratio ((Y₁+Y₂+Y)/X) is 0 ormore, and is preferably 1.0 or less.

(b) The co-crosslinking agent may contain other co-crosslinking agentthan the compound represented by the general formula (1) if theinventive effect is not impaired. Examples of the other co-crosslinkingagent include an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof. The α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or the metal salt thereof has an actionof crosslinking a rubber molecule by graft polymerization to a baserubber molecular chain. Examples of the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms include acrylic acid, methacrylic acid,fumaric acid, maleic acid, and crotonic acid.

Examples of the metal constituting the metal salt of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms include a monovalent metalion such as sodium, potassium and lithium; a divalent metal ion such asmagnesium, calcium, zinc, barium and cadmium; a trivalent metal ion suchas aluminum; and other ion such as tin and zirconium. The metalcomponent may be used solely, or two or more of them may be used incombination. Among them, as the metal component, the divalent metal suchas magnesium, calcium, zinc, barium and cadmium is preferable. This isbecause if the divalent metal salt of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms is used, a metal crosslinking betweenthe rubber molecules easily forms.

In a preferable embodiment according to the present invention, (b) theco-crosslinking agent preferably contains (meth)acrylic acid or themetal salt thereof and the compound represented by the general formula(1), and more preferably contains the metal salt of (meth)acrylic acidand the compound represented by the general formula (1).

The amount of (b) the co-crosslinking agent in the rubber composition ispreferably 10 parts by mass or more, more preferably 15 parts by mass ormore, and even more preferably 20 parts by mass or more, and ispreferably 50 parts by mass or less, more preferably 45 parts by mass orless, and even more preferably 40 parts by mass or less, with respect to100 parts by mass of (a) the base rubber. If the amount of (b) theco-crosslinking agent is less than 10 parts by mass, the amount of (c)the crosslinking initiator which will be described later must beincreased such that the constituent member formed from the rubbercomposition has an appropriate hardness, which tends to lower theresilience of the crosslinked rubber molded product. On the other hand,if the amount of (b) the co-crosslinking agent is more than 50 parts bymass, the constituent member formed from the rubber composition tends tobecome excessively hard.

The amount of the compound represented by the general formula (1) in therubber composition is preferably 3 parts by mass or more, morepreferably 3.5 parts by mass or more, and even more preferably 4 partsby mass or more, and is preferably 40 parts by mass or less, morepreferably 35 parts by mass or less, and even more preferably 30 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber. If the amount of the compound represented by the general formula(1) falls within the above range, the spin rate on driver shots isfurther decreased.

((c) Crosslinking Initiator)

(c) The crosslinking initiator is blended to crosslink (a) the baserubber component. As (c) the crosslinking initiator, an organic peroxideis suitable. Specific examples of the organic peroxide include a dialkylperoxide, a peroxy ester, a peroxy ketal, and a hydroperoxide. Examplesof the dialkyl peroxide include di(2-t-butylperoxyisopropyl) benzene(175.4° C.), dicumyl peroxide (175.2° C.),2,5-dimethyl-2,5-di(t-butylperoxy) hexane (179.8° C.), t-butylcumylperoxy (173.3° C.), di-t-hexyl peroxy (176.7° C.), di-t-butyl peroxy(185.9° C.), and 2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3 (194.3°C.). Examples of the peroxy ester include t-butylperoxy maleate (167.5°C.), t-butylperoxy-3,3,5-trimethyl cyclohexanoate (166.0° C.),t-butylperoxy laurate (159.4° C.), t-butylperoxyisopropyl monocarbonate(158.8° C.), t-hexylperoxy benzoate (160.3° C.),2,5-dimethyl-2,5-di(benzoylperoxy) hexane (158.2° C.), t-butylperoxyacetate (159.9° C.), and t-butylperoxy benzoate (166.8° C.). Examples ofthe peroxy ketal include 1,1-di(t-hexylperoxy)-3,3,5-trimethylcyclohexane (147.1° C.), 1,1-di(t-hexylperoxy) cyclohexane (149.2° C.),1,1-di(t-butylperoxy)-2-methyl cyclohexane (142.1° C.),1,1-di(t-butylperoxy) cyclohexane (153.8° C.), 2,2-di(t-butylperoxy)butane (159.9° C.), n-butyl-4,4-di(t-butylperoxy) valerate (172.5° C.),and 2,2-di(4,4-di(t-butylperoxy)cyclohexyl) propane (153.8° C.).Examples of the hydroperoxide include p-menthane hydroperoxide (199.5°C.), and diisopropylbenzene hydroperoxide (232.5° C.). The numericalvalue described in the parenthesis after the compound name of the aboveorganic peroxide is its one-minute half-life temperature. Among them,the dialkyl peroxide and the peroxy ketal are preferable. These organicperoxides may be used solely, or two or more of them may be used incombination. It is noted that when two or more of the above organicperoxides are used in combination, the difference between the maximumvalue and the minimum value of the one-minute half-life temperatures ofthe used organic peroxides is preferably 25° C. or less, more preferably10° C. or less.

The amount of (c) the crosslinking initiator is preferably 0.2 part bymass or more, more preferably 0.5 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.5 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe amount of (c) the crosslinking initiator is less than 0.2 part bymass, the constituent member formed from the rubber composition is sosoft that the resilience of the golf ball tends to be lowered, and ifthe amount of (c) the crosslinking initiator is more than 5.0 parts bymass, the amount of (b) the co-crosslinking agent described above mustbe decreased such that the constituent member formed from the rubbercomposition has an appropriate hardness, which tends to lower theresilience or worsen the durability of the golf ball.

((d) Metal Compound)

When the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms isused as the co-crosslinking agent, the rubber composition preferablyfurther contains (d) a metal compound. (d) The metal compound is notparticularly limited, as long as the metal compound is capable ofneutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms blended as the co-crosslinking agent in the rubber composition.Examples of (d) the metal compound include a metal hydroxide such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;a metal oxide such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and a metal carbonate such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. As (d) the metal compound, the divalent metalcompound is preferable, the zinc compound is more preferable. This isbecause the divalent metal compound reacts with the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms to form a metal crosslinking.In addition, if the zinc compound is used, the obtained golf ball hasbetter resilience. (d) The metal compound may be used solely, or atleast two of them may be used in combination.

[(e) Organic Sulfur Compound]

The rubber composition may further contain (e) an organic sulfurcompound. Examples of (e) the organic sulfur compound include at leastone member selected from the group consisting of thiophenols,thionaphthols, polysulfides, thiurams, thiocarboxylic acids,dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, andtheir metal salts. In the viewpoint of increasing the hardnessdistribution of the spherical core, as (e) the organic sulfur compound,the organic sulfur compound having the thiol group (—SH), or the metalsalt thereof is preferable, and thiophenols, thionaphthols or theirmetal salts are preferable.

Examples of the thiols include thiophenols and thionaphthols. Examplesof the thiophenols include thiophenol; thiophenols substituted with afluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol,2,6-difluorothiophenol, 2,4,5-trifluorothiophenol,2,4,5,6-tetrafluorothiophenol and pentafluorothiophenol; thiophenolssubstituted with a chloro group, such as 2-chlorothiophenol,4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,2,4,5,6-tetrachlorothiophenol and pentachlorothiophenol; thiophenolssubstituted with a bromo group, such as 4-bromothiophenol,2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol and pentabromothiophenol; thiophenolssubstituted with an iodo group, such as 4-iodothiophenol,2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol and pentaiodothiophenol; and metal saltsthereof. As the metal salt, zinc salt is preferable.

Examples of the thionaphthols (naphthalenethiols) include2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol,2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol,1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol,1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol,2-acetyl-1-thionaphthol, and metal salts thereof. Among them,2-thionaphthol, 1-thionaphthol, and metal salts thereof are preferable.As the metal salt, a divalent metal salt is preferable, zinc salt ismore preferable. Specific examples of the metal salt include zinc saltof 1-thionaphthol and zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,and examples thereof include disulfides, trisulfides, and tetrasulfides.As the polysulfides, diphenyl polysulfides are preferable.

Examples of the diphenyl polysulfides include diphenyl disulfide;diphenyl disulfides substituted with a halogen group, such asbis(4-fluorophenyl) disulfide, bis(2,5-difluorophenyl) disulfide,bis(2,6-difluorophenyl) disulfide, bis(2,4,5-trifluorophenyl) disulfide,bis(2,4,5,6-tetrafluorophenyl) disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl) disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl) disulfide,bis(pentachlorophenyl) disulfide, bis(4-bromophenyl) disulfide,bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl) disulfide,bis(2,4,5-tribromophenyl) disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl) disulfide, bis(4-iodophenyl) disulfide,bis(2,5-diiodophenyl) disulfide, bis(2,6-diiodophenyl) disulfide,bis(2,4,5-triodophenyl) disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide and bis(pentaiodophenyl) disulfide; and diphenyl disulfidessubstituted with an alkyl group, such as bis(4-methylphenyl) disulfide,bis(2,4,5-trimethylphenyl) disulfide, bis(pentamethylphenyl) disulfide,bis(4-t-butylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide,and bis(penta-t-butylphenyl) disulfide.

Examples of the thiurams include thiuram monosulfides such astetramethylthiuram monosulfide; thiuram disulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide andtetrabutylthiuram disulfide; and thiuram tetrasulfides such asdipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylicacids include naphthalene thiocarboxylic acid. Examples of thedithiocarboxylic acids include naphthalene dithiocarboxylic acid.Examples of the sulfenamides include N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

(e) The organic sulfur compound may be used solely, or two or more ofthem may be used in combination. As (e) the organic sulfur compound, thethiophenols and/or the metal salts thereof, the thionaphthols and/or themetal salts thereof, the diphenyl disulfides, and the thiuram disulfidesare preferable, 2,4-dichlorothiophenol, 2,6-difluorothiophenol,2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol,2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol,2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl) disulfide,bis(2,6-dichlorophenyl) disulfide, bis(2,6-dibromophenyl) disulfide,bis(2,6-diiodophenyl) disulfide, and bis(pentabromophenyl) disulfide aremore preferable.

The amount of (e) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe amount of (e) the organic sulfur compound is less than 0.05 part bymass, the effect of adding (e) the organic sulfur compound may not beobtained, and the resilience of the golf ball may not be enhanced. Inaddition, if the amount of (e) the organic sulfur compound is more than5.0 parts by mass, the obtained golf ball has a great compressiondeformation amount and thus the resilience thereof may be lowered.

((f) Carboxylic Acid and/or Salt Thereof)

The rubber composition may contain (f) a carboxylic acid and/or a saltthereof. If (f) the carboxylic acid and/or the salt thereof iscontained, the obtained spherical core can have a greater outer-hard andinner-soft degree. Examples of (f) the carboxylic acid and/or the saltthereof include an aliphatic carboxylic acid, an aliphatic carboxylicacid salt, an aromatic carboxylic acid, and an aromatic carboxylic acidsalt. (f) The carboxylic acid and/or the salt thereof may be usedsolely, or two or more of them may be used in combination.

The aliphatic carboxylic acid may be either a saturated aliphaticcarboxylic acid (hereinafter sometimes referred to as “saturated fattyacid”) or an unsaturated aliphatic carboxylic acid (hereinaftersometimes referred to as “unsaturated fatty acid”). In addition, thealiphatic carboxylic acid may have a branched or cyclic structure. Thenumber of carbon atoms included in the saturated fatty acid ispreferably 1 or more, and is preferably 30 or less, more preferably 18or less, and even more preferably 13 or less. The number of carbon atomsincluded in the unsaturated fatty acid is preferably 5 or more, morepreferably 7 or more, and even more preferably 8 or more, and ispreferably 30 or less, more preferably 18 or less, and even morepreferably 13 or less. It is noted that (f) the carboxylic acid and/orthe salt thereof excludes (b) the α,β-unsaturated carboxylic acid having3 to 8 carbon atoms and/or the metal salt thereof used as theco-crosslinking agent.

Examples of the aromatic carboxylic acid include a carboxylic acidhaving a benzene ring in the molecule, and a carboxylic acid having anaromatic heterocycle in the molecule. The aromatic carboxylic acid maybe used solely, or two or more of them may be used in combination.Examples of the carboxylic acid having the benzene ring include anaromatic carboxylic acid having a carboxyl group directly bonding to abenzene ring, an aromatic-aliphatic carboxylic acid having an aliphaticcarboxylic acid bonding to a benzene ring, a polynuclear aromaticcarboxylic acid having a carboxyl group directly bonding to a fusedbenzene ring, and a polynuclear aromatic-aliphatic carboxylic acidhaving an aliphatic carboxylic acid bonding to a fused benzene ring.Examples of the carboxylic acid having the aromatic heterocycle includea carboxylic acid having a carboxyl group directly bonding to anaromatic heterocycle.

As the aliphatic carboxylic acid salt or aromatic carboxylic acid salt,salts of the above mentioned aliphatic carboxylic acid or aromaticcarboxylic acid can be used. Examples of the cation component of thesesalts include a metal ion, an ammonium ion, and an organic cation.Examples of the metal ion include a monovalent metal ion such as sodium,potassium, lithium and silver; a divalent metal ion such as magnesium,calcium, zinc, barium, cadmium, copper, cobalt, nickel and manganese; atrivalent metal ion such as aluminum and iron; other ion such as tin,zirconium and titanium. The cation component may be used solely, or twoor more of them may be used in combination.

The organic cation is a cation having a carbon chain. The organic cationis not particularly limited, and examples thereof include an organicammonium ion. Examples of the organic ammonium ion include a primaryammonium ion such as stearyl ammonium ion, hexyl ammonium ion, octylammonium ion and 2-ethylhexyl ammonium ion; a secondary ammonium ionsuch as dodecyl(lauryl) ammonium ion and octadecyl(stearyl) ammoniumion; a tertiary ammonium ion such as trioctyl ammonium ion; and aquaternary ammonium ion such as dioctyldimethyl ammonium ion anddistearyldimethyl ammonium ion. These organic cations may be usedsolely, or two or more of them may be used in combination.

Examples of the aliphatic carboxylic acid and/or the salt thereofinclude a saturated fatty acid and/or a salt thereof, and an unsaturatedfatty acid and/or a salt thereof. The saturated fatty acid and/or thesalt thereof is preferable, caprylic acid (octanoic acid), pelargonicacid (nonanoic acid), capric acid (decanoic acid), lauric acid, myristicacid, palmitic acid, stearic acid, behenic acid, and their potassiumsalt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt,copper salt, nickel salt and cobalt salt, are preferable. As theunsaturated fatty acid and/or the salt thereof, palmitoleic acid, oleicacid, linoleic acid, arachidonic acid, and their potassium salt,magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt,copper salt, nickel salt and cobalt salt, are preferable.

As the aromatic carboxylic acid and/or the salt thereof, benzoic acid,butylbenzoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoicacid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoicacid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid,biphenylcarboxylic acid, naphthalenecarboxylic acid,anthracenecarboxylic acid, furancarboxylic acid, thenoic acid, and theirpotassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt,iron salt, copper salt, nickel salt and cobalt salt, are particularlypreferable.

The amount of (f) the carboxylic acid and/or the salt thereof, forexample, is preferably 0.5 part by mass or more, more preferably 1.0part by mass or more, and even more preferably 1.5 parts by mass ormore, and is preferably 40 parts by mass or less, more preferably 35parts by mass or less, and even more preferably 30 parts by mass orless, with respect to 100 parts by mass of (a) the base rubber. If theamount of (f) the carboxylic acid and/or the salt thereof is 0.5 part bymass or more, the spherical core have a greater outer-hard andinner-soft degree, and if the amount of (f) the carboxylic acid and/orthe salt thereof is 40 parts by mass or less, lowering in the corehardness is suppressed, and thus the resilience is better.

The rubber composition may further contain an additive such as apigment, a filler for adjusting weight or the like, an antioxidant, apeptizing agent, and a softener, where necessary. In addition, therubber composition may contain a rubber powder obtained by pulverizing agolf ball core or offcuts produced when preparing a core.

Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment. As the whitepigment, titanium oxide is preferably used. The type of titanium oxideis not particularly limited, but rutile type is preferably used becauseof the high opacity. In addition, the amount of titanium oxide ispreferably 0.5 part by mass or more, more preferably 2 parts by mass ormore, and is preferably 8 parts by mass or less, more preferably 5 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber.

It is also preferred that the rubber composition contains both a whitepigment and a blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. In addition, examples of thepurple pigment include anthraquinone violet, dioxazine violet, andmethyl violet.

The filler blended in the rubber composition is mainly used as a weightadjusting agent for adjusting the weight of the golf ball obtained as afinal product, and may be blended where necessary. Examples of thefiller include an inorganic filler such as zinc oxide, barium sulfate,calcium carbonate, magnesium oxide, tungsten powder, and molybdenumpowder.

The amount of the antioxidant is preferably 0.1 part by mass or more andis preferably 1 part by mass or less with respect to 100 parts by massof (a) the base rubber. In addition, the amount of the peptizing agentis preferably 0.1 part by mass or more and is preferably 5 parts by massor less with respect to 100 parts by mass of (a) the base rubber.

[Preparation of Rubber Composition]

The rubber composition used in the present invention can be obtained bymixing and kneading (a) the base rubber, (b) the co-crosslinking agent,(c) the crosslinking initiator, and other optional additives or the likeused where necessary. The kneading method is not particularly limited.For example, the kneading can be conducted with a conventional kneadingmachine such as a kneading roll, a banbury mixer and a kneader.

Next, the step of heat-pressing the rubber composition to form thespherical core will be explained. First, the rubber composition isextruded into a rod shape with an extruding machine and cut into apredetermined length, to prepare a preliminarily molded product (alsoreferred to as “plug”). In the case that an extruding machine is usedfor the preparation of the plug, the rubber composition during kneadingmay be heated, but the heating temperature is preferably 75° C. or less.Alternatively, the plug can be prepared by molding the core rubbercomposition into a sheet shape having a certain thickness and punchingthe sheet shaped core rubber composition. The size of the plug can beappropriately varied depending on the size of the mold for compressionmolding. The obtained plug is preferably, for example, immersed into ananti-adhesion agent liquid such that the plug does not adhere to eachother, and aged for about 8 to 48 hours after being dried.

Subsequently, the plug is charged into the mold for molding the core andpress-molded. In the production method according to the presentinvention, in the step of heat-pressing the rubber composition to formthe spherical core, the rubber composition is preferably heat-pressed ata pressing temperature in a range of from t-60° C. to t-15° C. to formthe spherical core, when the one-minute half-life temperature of (c) thecrosslinking initiator is referred to as t ° C. If the heat-pressingtemperature falls within the above range, the hardness at the pointlocated at the distance of 37.5% of the core radius from the center ofthe obtained spherical core can be selectively lowered, and the spinrate on driver shots can be decreased. It is noted that the pressingtemperature in the present invention is a set temperature of thepress-molding machine.

The heat-pressing temperature is preferably t-60° C. or more, morepreferably t-50° C. or more, and even more preferably t-40° C. or more,and is preferably t-15° C. or less, more preferably t-20° C. or less. Ifthe heat-pressing temperature is t-60° C. or more, the outer-hard andinner-soft degree of the core can be increased, and if the heat-pressingtemperature is t-15° C. or less, an optimal hardness distribution isobtained, and thus the spin rate decrease effect is greater. It is notedthat when the rubber composition contains two or more of (c) thecrosslinking initiators, the heat-pressing temperature is necessarilyadjusted such that the heat-pressing temperature satisfies the aboverange regarding the one-minute half-life temperatures of all of (c) thecrosslinking initiators.

The heat-pressing temperature is preferably 120° C. or more, morepreferably 130° C. or more, and is preferably 170° C. or less. Themolding time is preferably 10 minutes or more, more preferably 12minutes or more, and even more preferably 15 minutes or more, and ispreferably 60 minutes or less, more preferably 50 minutes or less, andeven more preferably 45 minutes or less. In addition, the pressure atthe molding preferably ranges from 2.9 MPa to 11.8 MPa.

The center hardness Ho of the spherical core is preferably 30 or more,more preferably 35 or more, and even more preferably 40 or more in ShoreC hardness. If the center hardness Ho of the spherical core is 30 ormore in Shore C hardness, the spherical core is not excessively soft,and thus the resilience is better. In addition, the center hardness Hoof the spherical core is preferably 70 or less, more preferably 65 orless, and even more preferably 60 or less in Shore C hardness. If thecenter hardness Ho of the spherical core is 70 or less in Shore Chardness, the spherical core is not excessively hard, and thus the shotfeeling is better.

The surface hardness Hs of the spherical core is preferably 65 or more,more preferably 70 or more, and even more preferably 72 or more, and ispreferably 100 or less, more preferably 95 or less, and even morepreferably 90 or less in Shore C hardness. If the surface hardness ofthe spherical core is 65 or more in Shore C hardness, the spherical coreis not excessively soft, and thus the resilience is better. In addition,if the surface hardness of the spherical core is 100 or less in Shore Chardness, the spherical core is not excessively hard, and thus the shotfeeling is better.

The hardness difference (Hs−Ho) between the surface hardness Hs of thespherical core and the center hardness Ho of the spherical core ispreferably 0 or more, more preferably 10 or more, even more preferably15 or more, and most preferably 20 or more, and is preferably 60 orless, more preferably 55 or less, and even more preferably 50 or less inShore C hardness. If the hardness difference is great, the obtained golfball has a high launch angle and a low spin rate, and thus travels agreat flight distance.

The spherical core formed from the rubber composition used in thepresent invention has the low hardness H37.5 at the point located at thedistance of 37.5% of the core radius from the center of the sphericalcore. If the spherical core having the low H37.5 is used, the obtainedgolf ball has a decreased spin rate on driver shots. From thisviewpoint, the hardness H37.5 at the point located at the distance of37.5% of the core radius from the center of the spherical core ispreferably 40 or more, more preferably 45 or more, and even morepreferably 50 or more, and is preferably 75 or less, more preferably 70or less, and even more preferably 69 or less in Shore C hardness.

The ratio ((H37.5−Ho)/(Hs−Ho)) of the hardness difference (H37.5−Ho)between the hardness H37.5 at the point located at the distance of 37.5%of the core radius from the center of the spherical core and the centerhardness Ho of the spherical core to the hardness difference (Hs−Ho)between the center hardness Ho of the spherical core and the surfacehardness Hs of the spherical core is preferably 0.1 or more, morepreferably 0.2 or more, and is preferably 0.40 or less, more preferably0.38 or less, and even more preferably 0.36 or less. The ratio((H37.5−Ho)/(Hs−Ho) represents a relative hardness of the hardness H37.5at the point located at the distance of 37.5% of the core radius fromthe core center to the hardness difference (Hs−Ho) between the surfacehardness and the center hardness. If the ratio (H37.5−Ho)/(Hs−Ho) fallswithin the above range, the spin rate on driver shots is furtherdecreased.

The diameter of the spherical core is preferably 34.8 mm or more, morepreferably 36.8 mm or more, and even more preferably 38.8 mm or more,and is preferably 42.2 mm or less, more preferably 41.8 mm or less, evenmore preferably 41.2 mm or less, and most preferably 40.8 mm or less. Ifthe diameter of the spherical core is 34.8 mm or more, the cover is notexcessively thick, and thus the resilience is better. On the other hand,if the diameter of the spherical core is 42.2 mm or less, the cover isnot excessively thin, and thus the cover functions better.

When the spherical core has a diameter in a range of from 34.8 mm to42.2 mm, the compression deformation amount (shrinking amount along thecompression direction) of the spherical core when applying a load from98 N as an initial load to 1275 N as a final load to the spherical coreis preferably 2.0 mm or more, more preferably 2.8 mm or more, and ispreferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling isbetter, and if the compression deformation amount is 6.0 mm or less, theresilience is better.

[Cover]

The cover of the golf ball is formed from a cover composition containinga resin component. Examples of the resin component include an ionomerresin, a thermoplastic polyurethane elastomer having a trade name of“Elastollan (registered trademark)” available from BASF Japan Ltd., athermoplastic polyamide elastomer having a trade name of “Pebax(registered trademark)” available from Arkema K. K., a thermoplasticpolyester elastomer having a trade name of “Hytrel (registeredtrademark)” available from Du Pont-Toray Co., Ltd., and a thermoplasticstyrene elastomer having a trade name of “TEFABLOC (registeredtrademark)” available from Mitsubishi Chemical Corporation.

The cover composition for forming the cover of the golf ball in thepresent invention preferably contains a thermoplastic polyurethaneelastomer or an ionomer resin as the resin component. It is alsopreferred that when the ionomer resin is used, a thermoplastic styreneelastomer is used in combination. The amount of the polyurethane orionomer resin in the resin component of the cover composition ispreferably 50 mass % or more, more preferably 60 mass % or more, andeven more preferably 70 mass % or more.

In addition to the resin component, the cover composition may furthercontain a pigment component such as a white pigment (e.g. titaniumoxide), a blue pigment and a red pigment, a weight adjusting agent suchas zinc oxide, calcium carbonate and barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial or fluorescent brightener, as long as they do not impair theperformance of the cover.

The amount of the white pigment (e.g. titanium oxide) is preferably 0.5part or more, more preferably 1 part or more, and is preferably 10 partsor less, more preferably 8 parts or less, with respect to 100 parts bymass of the resin component constituting the cover. If the amount of thewhite pigment is 0.5 part by mass or more, it is possible to impart theopacity to the resultant cover. In addition, if the amount of the whitepigment is more than 10 parts by mass, the durability of the resultantcover may deteriorate.

The slab hardness of the cover composition is preferably set inaccordance with the desired performance of the golf ball. For example,in case of a so-called distance golf ball which focuses on a flightdistance, the cover composition preferably has a slab hardness of 50 ormore, more preferably 55 or more in shore D hardness, and preferably hasa slab hardness of 80 or less, more preferably 70 or less in shore Dhardness. If the cover composition has a slab hardness of 50 or more,the obtained golf ball has a higher launch angle and a lower spin rateon driver shots and iron shots, and thus travels a greater distance. Inaddition, if the cover composition has a slab hardness of 80 or less,the obtained golf ball has better durability. Further, in case of aso-called spin golf ball which focuses on controllability, the covercomposition preferably has a slab hardness of less than 50 in Shore Dhardness, and preferably has a slab hardness of 20 or more, morepreferably 25 or more in shore D hardness. If the cover composition hasa slab hardness of less than 50 in Shore D hardness, the flight distanceon driver shots can be improved by the core of the present invention, aswell as the obtained golf ball readily stops on the green due to thehigh spin rate on approach shots. In addition, if the cover compositionhas a slab hardness of 20 or more in Shore D hardness, the abrasionresistance is enhanced. In case of a plurality of cover layers, the slabhardness of the cover composition constituting each layer may beidentical or different as long as the slab hardness of the covercomposition constituting each layer falls within the above range.

Examples of the method of molding the cover of the golf ball accordingto the present invention include a method which comprises molding thecover composition into a hollow shell, covering the core with aplurality of the hollow shells and performing compression molding(preferably a method which comprises molding the cover composition intoa hollow half-shell, covering the core with two of the half-shells andperforming compression molding); and a method which comprises injectionmolding the cover composition directly onto the core.

Concave portions called “dimples” are usually formed on the surface ofthe cover when the cover is molded. The total number of dimples ispreferably 200 or more and 500 or less. If the total number of dimplesis less than 200, the dimple effect is hardly obtained. On the otherhand, if the total number of dimples exceeds 500, the dimple effect ishardly obtained because the size of the respective dimple is small. Theshape (shape in a plan view) of the dimples formed on the coverincludes, without limitation, a circle; a polygonal shape such as aroughly triangular shape, a roughly quadrangular shape, a roughlypentagonal shape and a roughly hexagonal shape; and other irregularshape. These shapes may be employed solely, or at least two of them maybe employed in combination.

The thickness of the cover is preferably 4.0 mm or less, more preferably3.0 mm or less, and even more preferably 2.0 mm or less. If the coverhas a thickness of 4.0 mm or less, the resultant golf ball has betterresilience or shot feeling. The thickness of the cover is preferably 0.3mm or more, more preferably 0.5 mm or more, even more preferably 0.8 mmor more, and most preferably 1.0 mm or more. If the cover has athickness of less than 0.3 mm, the durability or wear resistance of thecover may be lowered. In the case that the golf ball comprises aplurality of cover layers, the total thickness of a plurality of coverlayers preferably falls within the above range. The golf ball bodyhaving the cover formed thereon is ejected from the mold, and ispreferably subjected to surface treatments such as deburring, cleaningand sandblast where necessary. In addition, if desired, a paint film ora mark may be formed.

The golf ball according to the present invention preferably has adiameter ranging from 40 mm to 45 mm. In light of satisfying theregulation of US Golf Association (USGA), the diameter is mostpreferably 42.67 mm or more. In light of prevention of air resistance,the diameter is more preferably 44 mm or less, and most preferably 42.80mm or less. In addition, the golf ball preferably has a mass of 40 g ormore and 50 g or less. In light of obtaining greater inertia, the massis more preferably 44 g or more, and most preferably 45.00 g or more. Inlight of satisfying the regulation of USGA, the mass is most preferably45.93 g or less.

When the golf ball according to the present invention has a diameter ina range of from 40 mm to 45 mm, the compression deformation amount(shrinking amount along the compression direction) of the golf ball whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the golf ball is preferably 2.0 mm or more, more preferably 2.4 mm ormore, even more preferably 2.5 mm or more, and most preferably 2.8 mm ormore, and is preferably 4.0 mm or less, more preferably 3.8 mm or less,and even more preferably 3.6 mm or less. If the compression deformationamount is 2.0 mm or more, the golf ball does not become excessivelyhard, and thus the shot feeling is better. On the other hand, if thecompression deformation amount is 4.0 mm or less, the resilience isbetter.

The construction of the golf ball according to the present invention isnot particularly limited, as long as the golf ball comprises a sphericalcore and at least one cover layer covering the spherical core. Thespherical core is preferably single layered. Unlike a multiple layeredcore, the single layered spherical core does not have an energy loss atthe interface of the multiple layered core when being hit, and thus hasbetter resilience. In addition, the cover has a construction composed ofat least one layer, and may have either a single layered construction ora multiple layered construction composed of at least two layers.Examples of the golf ball according to the present invention include atwo-piece golf ball composed of a spherical core and a single layeredcover disposed around the spherical core; a multi-piece golf ball(including a three-piece golf ball) composed of a spherical core and atleast two cover layers disposed around the spherical core; and a woundgolf ball composed of a spherical core, a rubber thread layer formedaround the spherical core and a cover disposed around the rubber threadlayer. The present invention can be suitably applied to any one of theabove golf balls.

The FIGURE is a partially cutaway cross-sectional view of a golf ball 1according to one embodiment of the present invention. The golf ball 1comprises a spherical core 2, and a cover 3 covering the spherical core2. A plurality of dimples 31 are formed on the surface of the cover.Other portions than the dimples 31 on the surface of the golf ball 1 arelands 32. The golf ball 1 is provided with a paint layer and a marklayer on an outer side of the cover 3, but these layers are notdepicted.

EXAMPLES

Next, the present invention will be described in detail by way ofexamples. However, the present invention is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present invention are included in the scope ofthe present invention.

[Evaluation Method] (1) Compression Deformation Amount (mm)

The deformation amount along the compression direction of the core(shrinking amount along the compression direction of the core), whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the core, was measured.

(2) Core Hardness Distribution (Shore C Hardness)

A type P1 auto loading durometer available from Kobunshi Keiki Co., Ltd.provided with a Shore C type spring hardness tester was used to measurethe hardness of the core. The Shore C hardness measured at the surfaceof the core was adopted as the surface hardness of the core. Inaddition, the core was cut into two hemispheres to obtain a cut plane,and the hardness at the central point of the cut plane and the hardnessat predetermined distances from the central point of the cut plane weremeasured. It is noted that the hardness of the core was measured at fourpoints at the predetermined distance from the central point of the cutplane, and the average value thereof was adopted as the hardness of thecore at the predetermined distance. The hardness was measured with anautomatic hardness tester (Digitest II, available from Bareiss company)using a testing device of “Shore C”.

(3) Spin Rate on Driver Shots (rpm)

A W #1 driver provided with a metal head (SRIXON Z745, loft angle: 8.5°,available from Sumitomo Rubber Industries, Ltd.) was installed on aswing robot M/C available from Golf Laboratories, Inc. The golf ball washit at a head speed of 50 m/sec, and the spin rate of the golf ballright after the hitting was measured. The measurement was conductedtwelve times for each golf ball, and the average value thereof wasadopted as the measurement value for that golf ball. It is noted thatthe spin rate of the golf ball immediately after the hitting wasmeasured by continuously taking a sequence of photographs of the hitgolf ball. The spin rate of each golf ball is shown as a difference fromthe spin rate of the golf ball No. 6.

[Preparation of Compound Represented by Formula (1) (Fatty Acid MetalSalt)] (1) Fatty Acid Metal Salt No. 1

81 g (1.0 mol) of zinc oxide and 400 mL of toluene were charged into a 2L separable flask. The liquid was stirred to obtain a suspension, and 75g (1.0 mol) of acrylic acid and 89 g (1.0 mol) of crotonic acid wereadded dropwise and mixed in the suspension. After the dropwise addition,the resultant mixture was allowed to react at 75° C. for 8 hours. Afterthe reaction, the solvent was removed to obtain the fatty acid metalsalt No. 1.

(2) Fatty Acid Metal Salt No. 2

8 g (0.1 mol) of zinc oxide and 40 mL of toluene were charged into a 200mL separable flask. The liquid was stirred to obtain a suspension, and7.5 g (0.1 mol) of acrylic acid and 10 g (0.1 mol) of 2,4-pentadienoicacid were added dropwise and mixed in the suspension. After the dropwiseaddition, the resultant mixture was allowed to react at 75° C. for 4hours. After the reaction, the solvent was removed to obtain the fattyacid metal salt No. 2.

[Production of Golf Ball] (1) Production of Core

According to the formulations shown in Table 1, the rubber compositionswere kneaded with a kneading roll, and heat-pressed in upper and lowermolds, each having a hemispherical cavity, for 20 minutes to 40 minutesto produce spherical cores having a diameter of 39.8 mm. The pressingtemperatures were shown in Table 1.

TABLE 1 Golf ball No. 1 2 3 4 5 6 7 8 Core Rubber BR730 100 100 100 100100 100 100 100 composition WHITE SEAL 5 5 5 5 5 5 5 5 ZN-DA90S 17.611.8 17.6 27 27 30 25 22.1 Fatty acid 11.3 22.6 — 6 18 — — 3.5 metalsalt No. 1 Fatty acid — — 9 — — — — — metal salt No. 2 Zinc stearate 1.12.3 0.9 0.6 1.8 — — 0.3 DCP 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Corepressing temperature (° C.) 140 140 140 170 170 170 140 140 Corehardness Core compression 3.43 3.32 3.67 3.42 3.85 3.16 3.47 3.18distribution deformation amount (Shore C) (mm) Core center hardness Ho51.4 45.4 54.1 54.7 44.7 60.9 62.7 60.3 Hardness at 12.5% point 53.248.5 55.6 64.1 53.9 69.2 63.2 62.9 of core radius Hardness at 25% point55.2 51.7 58.3 69.5 62.5 72.8 65.0 65.8 of core radius Hardness H37.5 at37.5% 58.6 55.1 61.7 70.8 65.9 73.5 67.0 69.6 point of core radiusHardness at 50% point 68.6 63.3 66.8 71.3 67.7 73.9 70.1 73.7 of coreradius Hardness at 62.5% point 74.3 72.2 70.7 70.6 66.9 73.5 72.7 77.0of core radius Hardness at 75% point 77.1 77.9 74.5 73.2 71.6 75.7 74.279.0 of core radius Core surface hardness Hs 77.8 78.4 78.4 85.4 87.289.3 78.1 77.5 H37.5—Ho 7.2 9.8 7.6 16.2 21.2 12.7 4.3 9.3 Hs—Ho 26.533.1 24.3 30.8 42.6 28.5 15.4 17.2 (H37.5—Ho)/(Hs—Ho) 0.27 0.30 0.310.53 0.50 0.45 0.28 0.54 Golf ball Spin rate on driver shots −100 −120−70 −30 −50 0 0 −10 performance (rpm)

The materials used in Table 1 are shown as follows.

BR730: high-cis polybutadiene rubber (cis-1,4 bond amount=96 mass %,1,2-vinyl bond amount=1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.))=55,molecular weight distribution (Mw/Mn)=3) available from JSR Corporation

ZN-DA90S: zinc acrylate (including 10 mass % of zinc stearate) availablefrom Nisshoku Techno Fine Chemical Co., Ltd.

DCP: dicumyl peroxide “Percumyl (registered trademark) D” (one-minutehalf-life temperature: 175.2° C.) available from NOF Corporation

WHITE SEAL: zinc oxide available from INDOLYSAGHT Co. Ltd.

Zinc stearate: available from Wako Pure Chemical Corporation (purity isat least 99%)

(2) Production of Cover and Production of Golf Ball

The cover material having the formulation shown in Table 2 was extrudedwith a twin-screw kneading type extruder to prepare the covercomposition in a pellet form. The conditions for extruding the covercomposition were a screw diameter of 45 mm, a screw rotational speed of200 rpm, and screw L/D=35, and the mixture was heated to 160 to 230° C.at the die position of the extruder. The obtained cover composition wasinjection molded onto the spherical core obtained above such that theformed cover had a thickness of 1.5 mm, to produce golf balls having thespherical core and the cover covering the core. Evaluation results ofthe obtained golf balls are shown in Table 1.

TABLE 2 Cover composition No. 1 Himilan 1555 40 Himilan 1605 20 HimilanAM7329 40 Titanium dioxide (A220) 3 JF-90 0.2 Hardness (Shore D) 63Formulation: parts by mass

The materials used in Table 2 are shown as follows.

Himilan 1555: Na neutralized ionomer available from Dow-MitsuiPolychemicals Co., Ltd.

Himilan 1605: Na neutralized ionomer available from Dow-MitsuiPolychemicals Co., Ltd.

Himilan AM7329: Zn neutralized ionomer available from Dow-MitsuiPolychemicals Co., Ltd.

Titanium dioxide: A-220 available from Ishihara Sangyo Kaisha, Ltd.

JF-90: light stabilizer available from Johoku Chemical Co., Ltd.

It can be seen from the results shown in Table 1 that the golf ballaccording to the present invention has a decreased spin rate on drivershots.

This application is based on Japanese Patent application No. 2019-174424filed on Sep. 25, 2019, the contents of which are hereby incorporated byreference.

1. A golf ball comprising a spherical core and at least one cover layercovering the spherical core, wherein the spherical core is formed from arubber composition containing (a) a base rubber, (b) a co-crosslinkingagent, and (c) a crosslinking initiator, and (b) the co-crosslinkingagent contains a compound represented by the formula (1):(R¹OCO)M(COOR²)m  (1) in the formula (1), M represents a metal atom, mrepresents 1 or 2, R¹ and R² are different from each other, represent analkenyl group having 2 to 30 carbon atoms or an alkynyl group having 2to 30 carbon atoms and have a carbon-carbon double bond or acarbon-carbon triple bond at an α-position carbon atom bonding to acarbonyl group, and when m is 2, two of R² may be identical to ordifferent from each other.
 2. The golf ball according to claim 1,wherein R¹ and R² each have 2 to 6 carbon atoms in the compoundrepresented by the formula (1).
 3. The golf ball according to claim 1,wherein R¹ has 2 to 3 carbon atoms in the compound represented by theformula (1).
 4. The golf ball according to claim 1, wherein R² has 3 to6 carbon atoms in the compound represented by the formula (1).
 5. Thegolf ball according to claim 1, wherein the metal atom (M) is at leastone metal atom selected from the group consisting of zinc, calcium,magnesium and aluminum in the compound represented by the formula (1).6. The golf ball according to claim 5, wherein R¹ is an ethenyl group(vinyl group) or isopropenyl group, R² is a 1-propenyl group,1,3-butadienyl group or 1,3-pentadienyl group, and the metal atom (M) iszinc in the compound represented by the formula (1).
 7. The golf ballaccording to claim 1, wherein the spherical core has a hardnessdifference (Hs−Ho) of 20 or more in Shore C hardness between a centerhardness Ho of the spherical core and a surface hardness Hs of thespherical core.
 8. The golf ball according to claim 1, wherein a ratioof a hardness difference (H37.5−Ho) between a hardness H37.5 at a pointlocated at a distance of 37.5% of a core radius from a center of thespherical core and a center hardness Ho of the spherical core to ahardness difference (Hs−Ho) between the center hardness Ho of thespherical core and a surface hardness Hs of the spherical core, is 0.4or less.
 9. The golf ball according to claim 1, wherein the sphericalcore has a center hardness Ho in a range of from 30 to 70 in Shore Chardness.
 10. The golf ball according to claim 1, wherein the sphericalcore has a surface hardness Hs in a range of from 65 to 100 in Shore Chardness.
 11. The golf ball according to claim 1, wherein the sphericalcore has a hardness H37.5 in a range of from 40 to 75 in Shore Chardness at a point located at a distance of 37.5% of a core radius froma center of the spherical core.
 12. The golf ball according to claim 1,wherein (b) the co-crosslinking agent further contains anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof in addition to the compound represented by theformula (1).
 13. The golf ball according to claim 12, wherein theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is at leastone member selected from the group consisting of acrylic acid,methacrylic acid, fumaric acid, maleic acid, and crotonic acid.
 14. Thegolf ball according to claim 1, wherein (b) the co-crosslinking agentcontains (meth)acrylic acid and/or a metal salt thereof and the compoundrepresented by the general formula (1).
 15. The golf ball according toclaim 1, wherein the rubber composition contains (b) the co-crosslinkingagent in an amount ranging from 10 parts by mass to 50 parts by masswith respect to 100 parts by mass of (a) the base rubber.
 16. The golfball according to claim 1, wherein the rubber composition contains thecompound represented by the formula (1) in an amount ranging from 3parts by mass to 40 parts by mass with respect to 100 parts by mass of(a) the base rubber.
 17. A golf ball comprising a spherical core and atleast one cover layer covering the spherical core, wherein the sphericalcore is formed from a rubber composition containing (a) a base rubber,(b) a co-crosslinking agent, and (c) a crosslinking initiator, and (b)the co-crosslinking agent contains a compound represented by the formula(1):(R¹OCO)M(COOR²)m  (1) in the formula (1), M represents at least onemetal atom selected from the group consisting of zinc, calcium,magnesium and aluminum, m represents 1 or 2, R¹ and R² are differentfrom each other, R¹ represents an alkenyl group having 2 to 3 carbonatoms, R² represents an alkenyl group having 3 to 6 carbon atoms, R¹ andR² have a carbon-carbon double bond at an α-position carbon atom bondingto a carbonyl group, and when m is 2, two of R² may be identical to ordifferent from each other.
 18. The golf ball according to claim 17,wherein (b) the co-crosslinking agent further contains anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof in addition to the compound represented by theformula (1).
 19. The golf ball according to claim 18, wherein R¹ is anethenyl group (vinyl group) or isopropenyl group, R² is a 1-propenylgroup, 1,3-butadienyl group or 1,3-pentadienyl group, and the metal atom(M) is zinc in the compound represented by the formula (1).
 20. The golfball according to claim 19, wherein a ratio of a hardness difference(H37.5−Ho) between a hardness H37.5 at a point located at a distance of37.5% of a core radius from a center of the spherical core and a centerhardness Ho of the spherical core to a hardness difference (Hs−Ho)between the center hardness Ho of the spherical core and a surfacehardness Hs of the spherical core, is 0.4 or less.